U.S. patent number 6,181,771 [Application Number 09/306,099] was granted by the patent office on 2001-01-30 for x-ray source with selectable focal spot size.
This patent grant is currently assigned to Siemens Aktiengesellschaft. Invention is credited to Erich Hell, Detlef Mattern, Peter Schardt.
United States Patent |
6,181,771 |
Hell , et al. |
January 30, 2001 |
X-ray source with selectable focal spot size
Abstract
An X-ray source has an emitter for the production of an electron
beam and an anode on which the electron beam strikes in an X-ray
focal spot, and a magnet system that produces a dipole field and a
quadrupole field that is superimposed on this dipole field, for
deflecting and focusing the electron beam onto the anode. In
addition, an arrangement is provided that operates together with
the magnet system for the adjustment of the size of the X-ray focal
spot. This arrangement, in order to set a desired size of the X-ray
focal spot, adjusts the quadrupole field in so that the X-ray focal
spot has a width corresponding to the desired size of the X-ray
focal spot, and supplies to the magnet system a wobble signal that
influences the dipole field, this wobble signal effecting a
periodic displacement of the electron beam in a direction
transverse to the extension of the width of the X-ray focal spot.
This gives the focal spot an effect length, resulting from the
deflection and measured in the direction of the deflection, to
achieve a particular ratio of the effective length to the width of
the X-ray focal spot.
Inventors: |
Hell; Erich (Erlandgen,
DE), Mattern; Detlef (Erlandgen, DE),
Schardt; Peter (Roettenbach, DE) |
Assignee: |
Siemens Aktiengesellschaft
(Munich, DE)
|
Family
ID: |
7866860 |
Appl.
No.: |
09/306,099 |
Filed: |
May 6, 1999 |
Foreign Application Priority Data
|
|
|
|
|
May 6, 1998 [DE] |
|
|
198 20 243 |
|
Current U.S.
Class: |
378/137; 378/119;
378/136 |
Current CPC
Class: |
H01J
35/305 (20130101); H01J 2235/162 (20130101) |
Current International
Class: |
H01J
35/30 (20060101); H01J 35/00 (20060101); H01J
035/30 () |
Field of
Search: |
;378/119,125,136,137
;313/364 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Bruce; David V.
Assistant Examiner: Hobden; Pamela R
Attorney, Agent or Firm: Schiff Hardin & Waite
Claims
We claim as our invention:
1. An X-ray source comprising:
an evacuated housing;
an anode disposed in said housing;
a cathode disposed in said housing which emits an electron beam
which proceeds along a beam path in said housing to strike said
anode in a focal spot, having a size defined by a length and a
width, on said anode from which X-rays are emitted;
a magnet system which produces a dipole field and a quadrupole
field superimposed on said dipole field, said beam path proceeding
through said dipole field and said quadrupole field, for deflecting
and focusing said electron beam; and
said magnet system including a control unit for selectively
adjusting the size of said focal spot on said anode so that said
focal spot has a ratio of said length to said width, said control
unit adjusting said quadrupole field to set said width of said
focal spot and producing a wobble signal to modify said dipole
field to periodically displace said electron beam on said anode in
a direction transverse to said width by a distance to give said
focal spot an effective length relative to said width to produce
said ratio.
2. An X-ray source as claimed in claim 1 wherein said control unit
includes an adjustment element which can be adjusted to select said
ratio.
3. An X-ray source as claimed in claim 1 wherein said ratio is
substantially equal to one, as seen from a direction opposite to a
primary propagation direction of said x-rays emitted from said
focal spot.
4. An X-ray source as claimed in claim 1 wherein said cathode
comprises a cathode which emits said electron beam with a
substantially circular cross-section.
5. An X-ray source as claimed in claim 1 wherein said anode
comprises a rotating anode having an anode edge which is beveled
relative to a primary direction of propagation of said X-rays, said
focal spot being disposed on said anode edge, and wherein said
width of said focal spot proceeds in a tangential direction of said
anode and wherein said effective length of said focal spot proceeds
in a radial direction of said anode.
6. An X-ray source as claimed in claim 5 wherein said control unit
comprises an adjustment element for selectively setting said
ratio.
7. An X-ray source as claimed in claim 5 wherein said ratio is
substantially equal to one, as seen from a direction opposite to a
primary propagation direction of said x-rays emitted from said
focal spot.
8. An X-ray source as claimed in claim 5 wherein said cathode
comprises a cathode which emits said electron beam with a
substantially circular cross-section.
9. An X-ray source as claimed in claim 5 wherein said evacuated
housing comprises a rotating bulb with said anode attached to said
rotating bulb.
10. An X-ray source as claimed in claim 9 wherein said cathode is
rigidly connected to said rotating bulb, and wherein said magnet
system surrounds said rotating bulb.
11. An X-ray source as claimed in claim 1 wherein said control unit
produces said wobble signal with a chronological curve for
producing an intensity distribution of said X-rays at said focal
spot along a radial direction of said anode having a predetermined
shape deviating from a Gaussian distribution.
12. An X-ray source as claimed in claim 11 wherein said wobble
signal has a chronological curve comprising a sawtooth curve.
13. A method for operating an X-ray source comprising the steps
of:
emitting an electron beam along a beam path from a cathode;
producing a dipole field with a quadrupole field superimposed
thereon with a magnet system and interacting said electron beam
with said dipole field and said quadrupole field to focus and
deflect said electron beam onto a focal spot on an anode to cause
X-rays to be emitted from said anode; and
modifying said dipole field with a wobble signal to periodically
displace said electron beam on said anode in a direction transverse
to said width by a distance to give said focal spot an effective
length relative to said width to produce a predetermined ratio of
said effective length to said width.
14. A method as claimed in claim 13 comprising selecting said ratio
from among a plurality of settable ratios.
15. A method as claimed in claim 13 wherein the step of modifying
said dipole field with a wobble signal comprises modifying said
dipole field with a wobble signal having a sawtooth curve.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an X-ray source of the type having
an electron emitter for the production of an electron beam, an
anode on which the electron beam strikes in an X-ray focal spot,
and a magnet system that produces a dipole field and a quadrupole
field superimposed thereon, for the deflection and focusing of the
electron beam onto the anode.
2. Description of the Prior Art
In an X-ray source known from U.S. Pat. No. 5,883,936, fashioned as
a rotating bulb source, a magnet system is provided for the
deflection and focusing of the electron beam, which emanates from
the electron emitter and has an initially circular cross-section.
For the production of an X-ray focal spot that is substantially
circular, seen opposite the primary direction of propagation of the
X-rays emanating from the X-ray focal spot, the quadrupole field is
selected such that it modifies the cross-section of the electron
beam emitted by the cathode, which initially has a circular
cross-section. This modification occurs in such a way that the
X-ray focal spot arising at the edge of the anode is elongated in
the radial direction due to the anode edge being beveled relative
to the primary direction of radiation of the X-ray radiation,
relative to the width of the electron beam measured in the
tangential direction (length-to-width ratio). This has in turn the
result that, as seen in the direction opposite the primary
direction of propagation of the X-rays emanating from the X-ray
focal spot, the extension of the electron beam in the radial
direction corresponds to the extension of the electron beam in the
tangential direction. The X-ray focal spot thus has substantially
circular shape, with the electron density of the electron beam
shortly before the X-ray focal spot being higher than immediately
adjacent to the cathode. The Gaussian distribution of the electrons
over the cross-section is, however, maintained.
Expensive measures would be necessary in order to enable an
adjustment of the size of the X-ray focal spot in such a rotating
bulb source, e.g. by means of a switching the largest
elongation.
The size of the focal spot could be adjusted in a known manner by
means of an adjustable focusing voltage applied to a focus cup that
surrounds the electron emitter. An electron emitter with a variable
emission surface alternatively could be used that could be
constructed as a flat or spiral emitter with several emission
surfaces, in particular concentrically arranged, which can be
activated individually or together, corresponding to the desired
size of the X-ray focal spot. This would have the advantage that
the type of drive would be maximally compatible with existing
generators. However, disadvantages would include higher
manufacturing costs and reduced flexibility. In addition, narrow
tolerances in the cathode manufacturing would have to be taken into
account.
In addition, it is disadvantageous that neither of the two
possibilities offers advantageous conditions for an optimization of
the intensity distribution of the X-rays emanating from the X-ray
focal spot in the sense of a rectangular curve of the intensity of
the X-rays in the radial direction of the X-ray focal spot.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an X-ray source of
the type described wherein several different sizes of the X-ray
focal spot are possible at low cost
According to the invention, this object is achieved in an X-ray
source having an electron emitter for the production of an electron
beam, having an anode on which the electron beam strikes in an
X-ray focal spot, and a magnet system that produces a dipole field
and a quadrupole field that is superimposed on this dipole field,
for the deflection and focusing of the electron beam, and an
arrangement that operates together with the magnet system for the
adjustment of the size of the X-ray focal spot. This arrangement,
in order to set a desired size of the X-ray focal spot, adjusts the
quadrupole field so that the X-ray focal spot has a width
corresponding to the desired size of the X-ray focal spot, and
supplies to the magnet system a wobble signal that influences the
dipole field. This wobble signal effects a periodic displacement of
the electron beam in a direction transverse to the extension of the
width of the X-ray focal spot over a distance such that the
effective length--resulting from the deflection and measured in the
direction of the deflection--of the X-ray focal spot is dimensioned
to achieve a particular ratio of the effective length to the width
of the X-ray focal spot.
Thus in the case of the inventive X-ray source the width of the
X-ray focal spot can be adjusted by influencing the quadrupole
field, and then, if the cross-section of the electron beam, with
respect to its ratio of length to width at the strike point of the
electron beam on the anode does not correspond to the desired ratio
of length to width of the X-ray focal spot, a dipole field is
influenced by a wobble signal so that the electron beam is
periodically deflected over such a distance and in such a direction
that an X-ray focal spot results with an effective length that
produces the desired ratio of length to width of the X-ray focal
spot.
In the invention, X-ray focal spots of different size thus can be
produced at low cost, since the only additional expenses is that
required for components to produce the wobble signal that
influences the dipole field.
In a preferred embodiment of the invention, the particular ratio of
effective length to width of the X-ray focal spot is adjustable.
Arbitrarily small ratios of effective length to width of the X-ray
focal spot thus are not possible, since for each width of the X-ray
focal spot there is a minimum length, since the cross-section of
the electron beam can be influenced by the quadrupole field only in
such a way that, together with the width of the cross-section of
the electron beam, the length of the cross-section of the electron
beam is also modified. As the width of the cross-section of the
electron beam increases the length of the cross-section of the
electron beam decreases.
Preferably, according to a variant of the invention a particular
ratio of effective length to width of the X-ray focal spot is
produced so that this ratio, as seen opposite to the primary
direction of propagation of the X-ray beam emanating from the
anode, is equal to one, since then a high image quality can be
achieved, This ratio of effective length to width of the X-ray
focal spot of one is of particular importance when the electron
emitter produces an electron beam with substantially circular
cross-section, since then the X-ray focal spot, as seen opposite
the primary direction of propagation of the X-rays, has a circular
shape that enables a further improved image quality.
If the X-ray source has a rotating anode with an anode edge that is
beveled relative to the primary direction of propagation of the
X-rays emanating from the anode, the X-ray focal spot being located
on this edge, then according to a variant of the invention the
width of the X-ray focal spot extends in the tangential direction
of the anode and the resulting length of the X-ray focal spot
extends in the radial direction of the anode. Advantageous imaging
conditions then exist.
In X-ray sources fashioned as rotating bulb sources, the invention
can be realized with a particularly low expense, since then a
magnet system that produces a dipole field with a superimposed
quadrupole field is present anyway.
According to a variant of the invention, the wobble signal exhibits
a chronological curve such that the intensity distribution of the
X-ray radiation emanating from the X-ray focal spot has, in the
direction of the deflection of the electron beam, a predetermined
shape that deviates from a Gaussian distribution and is preferably
rectangular. An intensity distribution of the X-rays that is
approximately rectangular in the direction of the resulting length
of the X-ray focal spot can be realized if the chronological curve
of the wobble signal is substantially a sawtooth function.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic side view, partly in section, of an inventive
X-ray source, without the protective housing which normally
surrounds the X-ray source and which contains a cooling agent.
FIG. 2a shows a cross-section of the electron beam emanating from
the cathode in the X-ray source of FIG. 1 for production of the
smallest X-ray focal spot, with curves respectively representing
the electron density distribution along the length and width of the
cross-section.
FIG. 2b shoes a view as seen directly above the anode of the
smallest X-ray focal spot on the surface of the anode, with
respective curves showing the X-ray intensity distribution along
the length and width of the focal spot as seen from directly above
the anode.
FIG. 2c is a view of the focal spot of FIG. 2b, as seen from a
direction opposite the primary direction of propagation of the
X-rays, with curves respectively representing the X-ray intensity
distribution along the length and the width of the focal spot, as
seen from the direction opposite the primary direction of
propagation of the X-rays.
FIG. 3a shows the cross-section of an electron beam emanating from
the cathode for production of a larger focal spot, with curves
respectively showing the electron density distribution along the
length and width.
FIG. 3b shows the cross-section of the electron beam of FIG. 3a
immediately before the electron beam strikes the anode surface,
together with curves respectively showing the electron density
distribution along the length and width.
FIG. 3c is a view of the X-ray focal spot on the surface of the
anode, as seen from directly above the anode, showing displacement
of the focal spot due to deflection of the electron beam by a
wobble signal, together with curves respectively representing the
X-ray intensity distribution along the effective length and width
of the focal spot.
FIG. 3d shows the cross-section of the focal spot, as seen from a
direction opposite the primary direction of propagation of the
X-rays, on the anode surface, together with curves respectively
representing the X-ray intensity distribution, as seen in the
direction opposite the primary direction of propagation of the
X-rays, along the effective length and width of the focal spot.
FIG. 4 shows the signal supplied to the magnet system of the X-ray
source of FIG. 1 for producing a dipole field with a wobble signal
superimposed thereon.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows an X-ray source 1 according to the invention
constructed as a rotating bulb source, having an insulated vacuum
housing 2 with a substantially cylindrical region 3 and a segment 4
that is connected thereto and that expands in the shape of a
truncated cone.
At the free end of the cylindrical region 3 of the vacuum housing
2, a cathode 5 is arranged as an electron emitter. The cathode 5 is
connected via slip rings 6 with a suitable source of heating
current, and is applied to a negative potential. A focusing
electrode 7 is allocated to the cathode 5, the electrode 7 serving
to set the size of the cross-section of the electron beam emitted
by the cathodes during operation. In FIG. 1, the electron beam is
designated 8.
In the specified embodiment, the cathode 5 and the electrode 7 have
a substantially rotationally symmetrical construction causing the
electron beam 8 emanating from the cathode 5 and corresponding to
the tube current to have a substantially circular cross-section in
the vicinity of the cathode 5, as shown in FIGS. 2a and 3a.
An anode 9 is provided at the end of the vacuum housing 2 opposite
the cathode 5. The anode 9 forms the termination of the vacuum
housing 2 which is evacuated in the interior. The anode 9 is in the
form of an anode dish 10 with a truncated-cone-shaped anode edge 11
that can be deposited with tungsten. The electron beam 8 is
incident on the anode edge 11 in an X-ray focal spot designated FS,
in order to produce X-rays.
A cooling liquid, indicated by the arrows 12, flows around the
anode 9, this liquid filling a protective housing (not shown) that
surrounds the vacuum housing 2, and that serves to dissipate the
thermal energy that arises in the production of the X-rays.
In an operating mode known as a single-pole operating mode, as
shown in FIG. 1, the anode 9, which is electrically insulated from
the cathode 5, is at ground potential. In two-pole operation, the
anode 9 is at a positive potential relative to the cathode 5. An
electrical field thus arises between the cathode 5 and the anode 8,
due to the tube voltage across these components, this field serving
to accelerate the electrons emitted by the cathode 5 in the
direction toward the anode 9.
The vacuum housing 2 and the anode 9 are constructed so as to be
substantially rotationally symmetrical in relation to a center 13.
The vacuum housing 2 with the cathode 5, together with the focusing
electrode 7 and the anode 9, is mounted in the protective housing
(not shown in FIG. 1) so as to be able to be rotated about the
center 13, by means of bearing elements 14, 15. Suitable drive
means (not shown in FIG. 1) are provided in order to set the
arrangement into rotation in the operation of the X-ray source
1.
In its cylindrical region 3, the vacuum housing 2 is surrounded by
a magnet system 16 that is fastened in the protective housing (not
shown in FIG. 1). The magnet system 16 accordingly does not rotate
with the vacuum housing 2 during operation. The magnet system 16 is
supplied with electrical signals, i.e. currents and/or voltages, by
an arrangement for setting the size of the X-ray focal spot FS,
namely a supply unit 19. These currents/voltages serve to produce a
dipole field as well as to produce a quadrupole field superimposed
on this dipole field.
The quadrupole field serves to focus the electron beam 8. Such
focusing is set by means of an adjusting element 20 of the supply
unit 19. The adjusting element 20 causes modification of the field
strength of the quadrupole field. The width, extending in the
tangential direction of the anode 9 and the anode edge 11, of the
X-ray focal spot FS can be set to a desired size by this field
strength modification. As a result of the quadrupole field, the
initially circular cross-section of electron beam 8 is changed.
The dipole field serves to deflect the electron beam 8 in such a
way that the X-ray focal spot FS arises at the desired location on
the anode edge 11. For this purpose, the dipole field has a
constant field component.
The dipole field serves additionally to deflect the electron beam 8
in the radial direction of the anode 9 and the anode edge 11 so
that the striking location of the electron beam 8 is periodically
displaced on the anode edge 11 by an amount (distance)
predetermined by the constant field component of the dipole field.
This distance is such that the effective length--measured in the
radial direction of the anode 9 and the anode edge 11, and thus in
the direction of the deflection--of the X-ray focal spot FS arising
as a result of the deflection is in a desired ratio to the
aforementioned focal spot width. This ratio is another adjusting
element 21 of the supply unit 19. The scale allocated to the
adjustment element 21 shows the values for the ratio of length to
width as seen in a direction opposite to the direction of
propagation of the X-rays.
In order to effect this periodic deflection, a wobble signal is
superimposed on the signal that is supplied to the magnet system 16
by the supply unit 19 in order to produce the constant component of
the dipole field. This superimposition produces a periodically
changing component of the dipole field, this component serving for
the periodic deflection of the electron beam 8. The amplitude of
the wobble signal, and thus the distance of the displacement of the
X-ray focal spot FS, is the actual parameter which is modified when
a particular ratio is set by the adjustment element 21 of the
supply unit 19.
Preferably, a ratio of effective length to width of the X-ray focal
spot FS is set such that, seen opposite the primary direction of
propagation (designated 17 in FIG. 1) of the X-rays emanating from
the anode edge 11, the ratio of resulting length to width of the
X-ray focal spot FS is equal to one, i.e., the ratio of effective
length to width of the X-ray focal spot FS corresponds to the scale
ratio of the oblique anode edge 11.
In the following, as an example the manner of functioning of the
X-ray source according to FIG. 1 is explained on the basis of FIGS.
2a to 3d.
If the smallest possible size of the X-ray spot FS, having a width
of e.g. 0.5 mm, is to be set, given a ratio of length to width of
one as seen opposite the direction of propagation of the X-rays
(the situation in FIGS. 2a, 2b and 2c), the adjusting element 20 is
set to its left stop (extreme) and the adjusting element 21 is set
to the value 1. The magnet system 16 is then driven by the supply
unit 19 for the production of a quadrupole field such that the
cross-section of the electron beam 8 (which in the absence of this
quadrupole field would for example, as illustrated in FIG. 2a, have
a width of approximately 0.75 mm at its striking location on the
anode edge 11) is deformed in the tangential direction by
interaction with the quadrupole field so that the cross-section of
the electron beam 8, as shown in FIG. 2b, has a width of only 0.5
mm at its striking location on the anode edge 11. The cross-section
of the electron beam 8 is thereby simultaneously elongated in the
radial direction by interaction with the quadrupole field, so that
the cross-section of the electron beam 8, as shown in FIG. 2b its
striking location on the anode edge 11 now has a length of, for
example, 4 mm. The ratio of length to width of the X-ray focal spot
FS thus results as 4 mm: 0.5 mm=8.
Given an angle between the primary direction of propagation 17 of
the X-rays and the beveled anode edge 11, which in the specified
embodiment is 8.degree., the X-ray focal spot FS', as seen opposite
the primary direction of propagation 17 of the X-rays, thus has a
width of approximately 0.5 mm and a length of approximately 0.56
mm. The ratio of effective length to width, as seen opposite the
primary direction of propagation 17 of the X-rays, thus results as
0.56 mm: 0.5 mm=1.12. It thus has approximately the value one, so
that the X-ray focal spot FS, as seen opposite the primary
direction of propagation 17 of the X-ray radiation, has an
essentially circular shape as shown in FIG. 2c.
In order to produce a larger X-ray focal spot FS with a width of
approximately 1 mm, the adjustment element 20 is set to a position
to cause the supply unit 19 to drive the magnet system 16 in order
to produce a quadrupole field such that the cross-section of the
electron beam 8 (which in the absence of this quadrupole field
would for example, as illustrated in FIG. 3a, again have a width of
approximately 0.75 mm at its striking location on the anode edge
11) is deformed in the tangential direction by interaction with the
quadrupole field so that the cross-section of the electron beam 8
at its striking location on the anode edge II now has a width of
1.0 mm, as shown in FIG. 3b. The cross-section of the electron beam
8 is thereby again elongated in the radial direction by interaction
with the quadrupole field, but as a result of the modified
quadrupole field this now occurs in such a way that the
cross-section of the electron beam 8, as shown in FIG. 3b, now has
a length of only 3.3 mm at its striking location on the anode edge
11.
Since the length of the electron beam 8 at its striking location on
the anode edge 11 is thus too small to produce an X-ray focal spot
FS in which the length and width are substantially equal as seen
opposite the primary direction of propagation 17 of the X-rays, the
supply unit 19 additionally drives the magnet system 16 with a
wobble signal which causes the dipole field to change periodically
so that a periodic deflection of the electron beam 8 takes place in
the radial direction, i.e. in the direction of the needed extension
of the length of the X-ray focal spot FS. This deflection takes
place with an amplitude so that, as a result of the deflection, an
X-ray focal spot FS arises whose length resulting from the
deflection is dimensioned such that a ratio of effective length to
width of the X-ray focal spot FS is present that has, as in the
case of the smallest possible X-ray focal spot FS, the value 8.
This means that the effective length of the X-ray focal spot FS has
to be 8 mm, and the distance d by which deflection has to take
place has to be 8 mm-3.3 mm=4.7 mm=d. As shown in FIG. 3c, an X-ray
focal spot FS is then produced that, given the angle of 8.degree.
between the primary direction of radiation 17 of the X-rays and the
anode edge 11, appears as a circular X-ray focal spot FS', as seen
opposite the primary direction of propagation 17 of the X-rays, and
which has a ratio of effective length to width, also as seen
opposite the primary direction of propagation of the X-rays, that
approximates the value 1 set by means of the adjusting element
21.
Data are stored in the supply unit 19 that correspond to the
signals to be supplied for driving the magnet system 16 for the
production of the quadrupole field, the constant field portion of
the dipole field, and the periodically changing field portion of
the dipole field, dependent on the size, set by the adjustment
element 20, of the X-ray focal spot FS, and on the ratio, set by
the adjustment element 21, of resulting length to width of the
X-ray focal spot FS, so that the supply unit 19 supplies the magnet
system 16 with the signals corresponding to the settings of the
adjusting elements 20 and 21.
If setting elements (not shown in FIG. 1) are provided for the tube
current and/or the tube voltage, the aforementioned data are also
stored as a function of tube current and/or the tube voltage.
Given X-ray focal spots produced by deflection of the electron beam
8, as in the prior art a Gauss distribution of the intensity of the
X-ray radiation is present in the direction of the width of the
X-ray focal spot FS, i.e. in the radial direction of the anode 9
and the anode edge 11. The intensity distribution of the X-ray
radiation in the direction of the length of the X-ray focal spot
FS, i.e. in the radial direction of the anode 9, however, depends
on the chronological curve of the wobble signal. If, as in the
specified embodiment, this corresponds essentially to a sawtooth
function, designated 18 in FIG. 4, the intensity distribution of
the X-ray radiation in the direction of the length of the X-ray
focal spot FS is, as can be seen from FIG. 4, approximately
rectangular. Such an intensity distribution yields advantages both
with respect to the achievable imaging quality and distribution of
the thermal loading of the anode 9, and the latter contributing to
a larger useful life of the anode 9.
Instead of a sawtooth-shaped wobble signal 18, other chronological
curves of the wobble signal can be provided according dependent on
particular applications, e.g. the sinusoidal curve 18' shown in
broken lines in FIG. 4.
The above-specified inventive X-ray source offers, in particular,
the following advantages. A lower technological outlay is required,
since only one electron emitter is necessary. The X-ray source has
easily achievable retrofitting compatibility, i.e., an inventive
X-ray source can be used in existing installations, since, in
contrast to the prior art, no additional adjustable focusing
voltage is required. Only one heating characteristic is required
for all sizes of the X-ray focal spot; for the larger X-ray focal
spots a tube (bulb) piston temperature results that is lower than
in the prior art. In order to modify existing conventional rotating
bulb sources so as to correspond to the invention, it is only
necessary to slightly modify the drive of the magnet system, since
the single change required for the realization of the invention is
the superimposition of a wobble signal and a different setting of
the quadrupole field. Rotating bulb sources thus can be modified
easily or improved in this way, which can be advantageous for the
modular construction of a group of sources. Lastly, the inventive
construction can be realized at low cost.
The electron emitter is designed so that both the smallest desired
size of the X-ray focal spot, as well as the required maximum tube
current, can be realized. In addition, it should be noted that
given small sizes of the X-ray focal spot, slightly higher bulb
temperatures can occur than in the prior art.
In the production of the smallest possible X-ray focal spot, in the
specified embodiment no wobble signal is supplied to the magnet
system 16, since a desired ratio of length to width is already
achieved without this wobble signal. This does not mean, however,
that a wobble signal cannot also be employed for the production of
the smallest possible X-ray focal spot.
In the embodiment specified above, a rotating bulb source is
provided as an X-ray source. The invention can also be used in
differently constructed X-ray sources, e.g. in a rotating-anode
X-ray source according to U.S. Pat. No. 5,812,632, or in
fixed-anode X-ray sources.
The size of the focal spot and the ratio of length to width of the
X-ray focal spot can be adjusted continuously in the specified
embodiment. It is also possible within the scope of the invention
to provide several switchable sizes of focal spots, with a fixed
ratio of length to width of each X-ray focal spot.
In the specified embodiment, the electron beam 8 emanating from the
electron emitter 5 has a circular cross-section. The invention can
also be used in connection with X-ray sources whose electron
emitters produce electron beams that have a cross-section that
deviates from a circular shape.
The oblique positioning, provided in the specified embodiment, of
the region of the anode in which the X-ray focal spot is located,
relative to the primary direction of propagation of the X-rays,
need not necessarily be present within the scope of the
invention.
Although modifications and changes may be suggested by those
skilled in the art, it is the intention of the inventors to embody
within the patent warranted hereon all changes and modifications as
reasonably and properly come within the scope of their contribution
to the art.
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